Contaminant getter on UV reflective base coat in fluorescent lamps

ABSTRACT

An electric lamp is provided having a luminescent layer on the lamp envelope that produces visible light when impinged by ultraviolet radiation generated within the lamp. An undercoat for the electric lamp increases the luminous efficacy of the lamp. The undercoat includes a particulate non-fluorescent material derived from a sintered mixture of an aluminum oxide material with a contiguous layer of getter material which reacts with contaminants present in the lamp. The getter material is an alkaline earth metal borate material or mixtures thereof.

This application relates to co-pending patent application Ser. No.10/017,360 filed Dec. 14, 2001 of Charles Trushell and Liviu Mageantitled “CONTAMINANT GETTER ON UV REFLECTIVE BASE COAT IN FLUORESCENTLAMPS” and assigned to the same assignee as in the present application.

This invention relates to low-pressure mercury vapor lamps, morecommonly known as fluorescent lamps, having a lamp envelope withphosphor coating, and more particularly, to such lamps in which theamount of contaminants introduced into the lamp during manufacture hasbeen reduced during lamp operation. This has the effect of reducingmercury consumption, improving maintained light output and improving arcstability at the time of lamp ignition.

Low-pressure mercury vapor lamps, more commonly known as fluorescentlamps, have a lamp envelope with a filling of mercury and rare gas tomaintain a gas discharge during operation. The radiation emitted by thegas discharge is mostly in the ultraviolet (UV) region of the spectrum,with only a small portion in the visible spectrum. The inner surface ofthe lamp envelope has a luminescent coating, often a blend of phosphors,which emits visible light when impinged by the ultraviolet radiation.

There is an increase in the use of fluorescent lamps because of reducedconsumption of electricity. To further reduce electricity consumption,there is a drive to increase efficiency of fluorescent lamps, referredto as luminous efficacy which is a measure of the useful light output inrelation to the energy input to the lamp, in lumens per watt (LPW).

U.S. Pat. No. 5,552,665 of Charles Trushell, the inventor in the presentapplication, relates to an electric lamp having a luminescent layer onthe lamp envelope which produces visible light when impinged byultraviolet radiation generated within the lamp, and wherein anundercoat for the luminescent layer is employed. The disclosure of saidpatent is hereby incorporated by this reference thereto. Such anundercoat is now a common feature of modern fluorescent lamps, and is anoxidic, particulate base coat layer of non-fluorescent material,preferably an aluminum oxide, underlying the light-giving phosphor. Suchan undercoat or base-coat is intended to economically increase lightoutput, simplify the manufacturing process, improve the maintenance oflight output, and reduce mercury consumption by the glass bulb.Typically, such layers are composed of very small particles withconsequently large surface areas. Unfortunately, it has been found thatthe large surface of the particulate base-coat combined with thepropensity of aluminum oxide to adsorb gaseous molecules results inlarger than normal amounts of contaminants being introduced into thelamp interior during manufacture. For example, water and carbon dioxideare common, volatile, fluorescent lamp contaminants, the amounts ofwhich are increased as a result of the large surface area of theundercoat. One effect of the increased amount of these contaminants isto increase the duration of arc instability immediately after lampignition.

It is also known to coat the phosphor layer contained in a fluorescentlamp. For example:

Tamura, Japanese Patent Application No. 03179238 (Abstract)), describesa procedure wherein MgO is mixed with a phosphor at 0.01-1.0% and usedto form a layer as a step in the manufacture of a fluorescent lamp inorder to getter CO₂ and CO impurities which exist after the lamp ismanufactured.

Watanabe et al, U.S. Pat. No. 5,604,396, describes a method wherein analcoholic solution of a metal alkoxide (wherein the metal may be any ofnumerous metals including magnesium) is added to an aqueous suspensionof a phosphor, which is to be coated by the alkoxide. Upon evaporationof the alcohol, the alkoxide is converted to a hydroxide andhomogeneously precipitated on the surface of the phosphor in a sol-gelprocess. After removal of the water, the hydroxide-coated phosphor isfired at a high temperature; however, no specific benefits are claimedfor coating the phosphor with the metal alkoxide. Moreover, coating thephosphor with metal alkoxide or metal oxide does not eliminate ormitigate the increase in duration of the arc instability in the lampwhen an oxidic base-coat such as alumina is used.

In said co-pending application Ser. No. 10/017,360 filed Dec. 14, 2001of Charles Trushell and Liviu Magean titled “CONTAMINANT GETTER ON UVREFLECTIVE BASE COAT IN FLUORESCENT LAMPS” referred to above, the needin the art for a means of reducing the amount of contaminants and foreliminating or at least mitigating the increase in duration of arcinstability to which the contaminants contribute in a fluorescent lampis addressed by providing an undercoat layer comprising a particulatenon-fluorescent material derived from a sintered mixture of an aluminumoxide material and, as a getter material which is capable ofirreversible reaction with contaminants present in the lamp, aparticulate oxidic material, preferably an aluminum oxide having on itssurface, preferably as a contiguous layer, an oxide of an alkaline earthmetal or zinc formed in situ during the lehring (sintering) process viareaction, for example, through thermal decomposition, of an alkalineearth metal oxide precursor material or zinc oxide precursor material ormixture thereof which reacts to form an alkaline earth metal oxide orzinc oxide or mixture thereof.

There is a continued need in the art for fluorescent lamps in which saidundercoat layer comprising a particulate non-fluorescent materialderived from a sintered mixture of an aluminum oxide material andalternative getter materials which are capable of irreversible reactionwith contaminants present in the lamp.

An object of the invention is to provide a fluorescent lamp in which theamount of contaminants is reduced and in which the arc instability towhich the contaminants contribute is substantially eliminated.

The present invention accomplishes the above and other objects byproviding an electric lamp that includes:

an envelope having an inner surface;

means within the lamp envelope for generating ultraviolet radiation;

a layer of a luminescent material adjacent to the inner surface of thelamp envelope for generating visible light when impinged by saidultraviolet radiation; and

an undercoat layer between said inner surface of said lamp envelope andsaid layer of luminescent material, for reflecting ultraviolet radiationwhich has passed through said layer of luminescent material back intosaid luminescent material for increasing the visible light output ofsaid luminescent material, said undercoat layer comprising a particulatenon-fluorescent material derived from a sintered mixture of an aluminumoxide material and an alkaline earth metal borate getter material whichis capable of irreversible reaction with contaminants present in thelamp.

In one embodiment of the invention, said undercoat layer comprises aparticulate oxidic material, preferably an aluminum oxide having on itssurface, preferably as a contiguous layer, a borate of an alkaline earthmetal formed in situ during the lehring (sintering) process viareaction, for example, through thermal decomposition, of an alkalineearth metal borate precursor material or mixture thereof which reacts toform an alkaline earth metal borate or mixture thereof on said oxidicbase-coat material.

In another embodiment, the undercoat layer comprises alumina having onits surface a contiguous layer of alkaline earth metal borate formed insitu during the lehring (sintering) process as a result of thermaldecomposition of an alkaline earth metal pyroborate precursor. In thisway advantage is taken of the large surface area of the oxidic base-coatmaterial, in part responsible for the arc instability, to act as thesite for said irreversible reaction.

Suitable getter materials are borates of alkaline earth metals andinclude magnesium, calcium, strontium, and barium borates, and mixturesthereof, formed in situ during the lehring (sintering) process by aprecursor compound or mixtures of such compounds which are introduced assoluble compounds into an aqueous suspension of the aluminum oxidebase-coat material. Mixtures forming magnesium pyroborates areparticularly preferred for use as a getter compound for purposes of thisinvention.

Suitable precursor materials may be any alkaline earth metal compound ormixture thereof that reacts during the lehring step to form an alkalineearth borate or mixture of such materials on the surface of the oxidicbase-coat material. Illustrative of such precursor materials suitablefor use herein are magnesium, calcium, strontium, and barium citrates,acetates, nitrates, etc. The preferred getter materials are pyroboratesof magnesium, calcium, strontium, and barium, and mixtures thereof,which are formed, for example, by addition of boric acid or ammoniumborate and calcium nitrate introduced as soluble compounds into thesuspension of the oxidic base coat material.

The FIGURE is a perspective view of a fluorescent lamp, partly incross-section, partly broken away, having an undercoat with gettermaterial according to the invention. The invention will be betterunderstood with reference to the details of specific embodiments thatfollow.

With reference to the FIGURE, there is illustrated a low-pressuremercury vapor discharge or fluorescent lamp 1 with an elongated outerenvelope, or bulb 3. The lamp includes a conventional electrodestructure 5 at each end which includes a filament 6 supported in in-leadwires 7 and 9 which extend through a glass press seal 11 in a mount stem10. The electrode structure 5 is not the essence of the presentinvention, and other structures may be used for lamp operation togenerate and maintain a discharge in the discharge space. For example, acoil positioned outside the discharge space may be used to generate analternating magnetic field in the discharge space for generating andmaintaining the discharge.

Returning to the illustrative lamp 1 of the FIGURE, the leads 7,9 areconnected to pin-shaped contacts 13 of their respective bases 12 fixedat opposite ends of the lamp 1. The discharge-sustaining fillingincludes an inert gas such as argon, or a mixture of argon and othergases, at a low pressure in combination with a small quantity of mercuryto sustain an arc discharge during lamp operation. The inner surface 15of the outer envelope 3 is provided with an undercoat 16 of aluminumoxide as a non-fluorescent material coated with a contiguous layer of analkaline earth borate formed as follows:

A mixture of boric acid or ammonium borate is added to a solution ofcalcium nitrate. Preferably, the borate-producing moiety (boric acid orammonium borate) and the alkaline earth metal nitrate (calcium nitrate)are present in a molar ratio such that about 4 moles of boron arepresent for each mole of alkaline earth metal. This favors theproduction of alkaline earth metal pyroborates which are believed to bethe borate form that is particularly beneficial in the elimination ofarc instability according to the invention. The individual borate saltsformed are believed to melt and thermally decompose to formpredominantly pyroborates during sintering. For this reason, mixtures ofalkaline earth metal nitrates or other soluble salts may be employedwith good advantage to achieve a desired composition while obtaining asufficiently low thermal decomposition temperature. The alkaline earthpyroborate formed from the proportions indicated above represents fromabout 1 to about 3 wt. % of borate based on the weight of the aluminumoxide as getter material to remove contaminants from the lamp.

Once the base-coat layer 16 is sintered, the resulting borate gettermaterial(s) become quite water-insoluble. This permits the applicationof an aqueous suspension of phosphors to form a phosphor layer 17directly over the non-fluorescent base-coat. After drying, this layer issintered again before it is made into a lamp.

A phosphor coating 17 is disposed over the undercoat 16. Both coatingsextend the full length of the bulb, completely circumferentially aroundthe bulb inner wall.

The undercoat layer may be cast from organic solvent or water basedsuspensions to which various components may be added withoutsubstantially changing the various advantages of the non-fluorescentoxidic undercoat. The suspension is applied to the interior of a cleanfluorescent tube in a manner known to the art and is then lehred orsintered, also in a manner well known in the art.

The bulb is then lehred and finished into a lamp in the manner known inthe art.

To further reduce mercury consumption, the glass mount stems and pressseals may also be coated with the aluminum oxide undercoat layer toreduce mercury bound to the glass mount stems and press seals.

This invention recognizes the discovery that alkaline earth metalborates, particularly when incorporated in aluminum oxide reflectiveundercoats via thermal decomposition of precursor materials duringlehring, are effective to reduce or eliminate contaminants introducedinto the lamp during manufacture and substantially reduces the durationof or eliminates arc instability immediately after lamp ignition.

The invention was demonstrated in a series of 32T8 fluorescent bulbs, 4feet in length and 1 inch in diameter using about 0.5-1.0 grams ofcommercially available aluminum oxide containing amounts of calciumnitrate and boric acid calculated to produce about 1-3% calciumpyroborate based on the weight of the aluminum oxide.

Representative lamps were produced in which the undercoat layer 16comprises particulate aluminum oxide, i.e. alumina having on its surfacea contiguous layer of an alkaline earth metal borate or phosphate. Thealumina was suspended in a water-based solution to which an amount ofboric acid and barium nitrate is added, and flushed down the lamp tubeor envelope 3 to flow over the envelope inner surface 15 until it exitsfrom the other end. The solution was dried in a drying chamber. Aphosphor coat 17 was applied in a similar fashion and sintered or bakedfor a period of time.

Several other lamps were made using the same procedure but without anyborate additives or borate precursor additives. Only the alumina UVreflecting base-coats were applied in these lamps since the arc in sucha lamp design is known to be particularly unstable once ignited. All ofthe lamps were produced and stored at room temperature unignited for 3months when they were examined for the relative arc instability. Theresults showed that in lamps of this invention, arc instability afterlamp ignition was virtually eliminated. In contrast, lamps that were notso processed and treated exhibited a substantial period of arcinstability after lamp ignition.

The phosphors suitable for use in this invention may vary according tothe properties desired in the final lamp. For example, for a 4100° Kfluorescent lamp where the color temperature is about 4100° K, i.e., indegree Kelvin, the phosphor coat 17 is typically comprised of a mixtureof three phosphors. The phosphor mixture typically consists of ablue-emitting barium magnesium aluminate (BAM) activated by Eu, ared-emitting yttrium Oxide (YOX) activated by Eu, i.e., Y₂O₃:Eu; andtypically a green-emitting lanthanum phosphate (LAP) activated by ceriumand terbium.

The three-phosphor mixture in the 4100° K lamp allows the lamp 1 to havereduced mercury consumption in conjunction with the alumina undercoat 16which shields the glass envelope 3 from mercury.

Since very thin layers of the getter compounds are effective ingettering the contaminants in question, the optics of the bulk materialare not effectively altered. The invention has been found to be usefulin all UV reflective base coats in fluorescent lamps.

While not wishing to be bound by any theory, experimental data indicatesthat contamination above a certain threshold level in the finished lampresults in a dramatically increased duration of arc instability inconventional lamps and that decreasing the contamination below thisthreshold dramatically reduces, and in most cases, virtually eliminatesthe duration of the arc instability. Thus the solution according to thisinvention is the reduction of impurities responsible for thecontamination by taking advantage of the large surface area provided bythe UV reflecting base-coat. It is believed that most of the waterpresent in the lamp will be reacted with borate getter material duringmanufacture of the lamp. As a result once the lamp is manufactured, theamount of water physisorbed on the particulate aluminum oxide will bereduced to an amount that is incapable of contributing to arcinstability.

While the present invention has been described in particular detail, itshould also be appreciated that numerous modifications are possiblewithin the intended spirit and scope of the invention. In interpretingthe appended claims it should be understood that where and if itappears:

-   -   a) the word “comprising” does not exclude the presence of        elements other than those listed in a claim;    -   b) the word “consisting” excludes the presence of elements other        than those listed in a claim;    -   c) the word “a” or “an” preceding an element does not exclude        the presence of a plurality of such elements; and    -   d) any reference signs in the claims do not limit their scope.

1. An electric lamp, comprising: a) a lamp envelope having an innersurface; b) means within the lamp envelope for generating ultravioletradiation; c) a layer of a luminescent material adjacent the innersurface of the lamp envelope for generating visible light when impingedby said ultraviolet radiation; and d) a base-coat layer, between saidinner surface of said lamp envelope and said layer of luminescentmaterial, for reflecting ultraviolet radiation which has passed throughsaid layer of luminescent material back into said luminescent materialfor increasing the visible light output of said luminescent material,said base-coat layer comprising a particulate non-fluorescent oxidicmaterial with a getter material on its surface which reacts withcontaminants present in the lamp, said getter material comprising analkaline earth metal pyroborate.
 2. The lamp according to claim 1,wherein said getter material is formed upon thermal decomposition of agetter precursor material during lehring.
 3. The lamp according to claim1, wherein said base-coat layer comprises a particulate aluminum oxidehaving a contiguous layer of a borate of an alkaline earth metal ormixtures thereof formed by exposing the particulate aluminum oxideduring the lehring process to an effective amount of a precursormaterial of an alkaline earth metal borate getter compound.
 4. The lampas claimed in claim 3, wherein said base-coat layer is sintered justprior to the envelope being sealed during manufacture of said lamp. 5.The lamp as claimed in claim 3, wherein said getter material includes aborate of an alkaline earth metal selected from the group consisting ofmagnesium, calcium, strontium, barium, and mixtures thereof.
 6. The lampas claimed in claim 5, wherein said sintered mixture is derived from asoluble precursor compound of an alkaline earth metal borate or mixturesthereof in an aqueous suspension of aluminum oxide.
 7. The lampaccording to claim 1, wherein said means for generating ultravioletradiation is disposed within said lamp envelope, said lamp envelopeincluding a filling of an ionizable material and a rare gas and a pairof discharge electrodes between which a discharge takes place duringlamp operation.
 8. The lamp according to claim 1, wherein said means forgenerating ultraviolet radiation comprises a filling of an ionizablematerial and a rare gas within said lamp envelope and a pair ofdischarge electrodes each adjacent to a respective sealed end of saiddischarge vessel.
 9. An electric lamp, comprising: a) a lamp envelopehaving an inner surface; b) means within the lamp envelope forgenerating ultraviolet radiation; c) a layer of a luminescent materialadjacent the inner surface of the lamp envelope for generating visiblelight when impinged by said ultraviolet radiation; and d) a base-coatlayer, between said inner surface of said lamp envelope and said layerof luminescent material, for reflecting ultraviolet radiation which haspassed through said layer of luminescent material back into saidluminescent material for increasing the visible light output of saidluminescent material, said base-coat layer comprising a particulatenon-fluorescent oxidic material with a getter material on its surfacewhich reacts with contaminants present in the lamp, said getter materialcomprising an alkaline earth metal borate or mixtures thereof, whereinsaid base-coat layer comprises a particulate aluminum oxide having acontiguous layer of the alkaline earth metal borate or mixtures thereofformed by exposing the particulate aluminum oxide during the lehringprocess to an effective amount of a precursor material of an alkalineearth metal borate getter compound, and wherein said getter materialcomprises an alkaline earth metal pyroborate.
 10. The lamp as claimed inclaim 9, wherein said layer of luminescent material comprises ahalophosphate phosphor.
 11. A low pressure mercury vapor fluorescentlamp, comprising: a) a tubular, light transmissive lamp envelope havingopposing sealed ends and an inner tubular surface; b) a filling ofmercury and a rare gas; c) a pair of discharge electrodes each arrangedat a respective sealed end of said lamp envelope; d) means forconnecting said discharge electrodes to a source of electric potentialoutside of said lamp envelope, whereby during lamp operation a gasdischarge is maintained between said discharge electrodes, wherein saidgas discharge emits ultraviolet radiation; e) a first layer disposed onsaid inner tubular surface of said lamp envelope, wherein said firstlayer is a light transmissive and ultraviolet radiation reflectinglayer, said first layer comprising a sintered mixture of an aluminumoxide material and a getter material which reacts with contaminantspresent in the lamp, said getter material comprising an alkaline earthmetal pyroborate; and f) a second layer of luminescent material disposedon said first layer.
 12. The lamp as claimed in claim 11, wherein saidfirst layer comprises a particulate aluminum oxide having a contiguouslayer of an alkaline earth metal borate or mixtures thereof formed byexposing the particulate aluminum oxide material during the lehringprocess to an effective amount of an alkaline earth metal borateprecursor compound.
 13. The lamp as claimed in claim 11, wherein saidfirst layer is sintered just prior to the envelope being sealed duringmanufacture of said lamp.
 14. The lamp as claimed in claim 13, whereinsaid alkaline earth metal is selected from the group consisting ofmagnesium, calcium, strontium, barium, and mixtures thereof.
 15. Thelamp as claimed in claim 14, wherein said sintered mixture is derivedfrom a soluble borate precursor material of said alkaline earth metal ormixture thereof in an aqueous suspension of aluminum oxide.
 16. Anelectric lamp, comprising: a) a lamp envelope having an inner surface;b) means within the lamp envelope for generating ultraviolet radiation;c) a layer of a luminescent material adjacent the inner surface of thelamp envelope for generating visible light when impinged by saidultraviolet radiation; and d) a base-coat layer, between said innersurface of said lamp envelope and said layer of luminescent material,for reflecting ultraviolet radiation which has passed through said layerof luminescent material back into said luminescent material forincreasing the visible light output of said luminescent material, saidbase-coat layer comprising a particulate non-fluorescent oxidic materialwith a getter material on its surface, said getter material comprisingan alkaline earth metal pyroborate effective to react with contaminantspresent in the lamp to the extent that arc instability after ignition ofthe lamp is substantially eliminated.
 17. The electric lamp as claimedin claim 16, wherein said base-coat is formed from material comprisingabout 0.5 to about 1.0 grams of aluminum oxide containing an amount ofcalcium nitrate and boric acid calculated to produce as said gettermaterial about 1 to about 3% calcium pyroborate based on the weight ofthe aluminum oxide.